1. Field
This invention relates to the field of spectrophotometry.
2. Description of Related Art
Automatic stand-alone color calibration systems employ spectrophotometers mounted in the path of a moving substrate, such as, for example, paper, moving in a path in marking devices, preferably in the output path of the marking device. In electrophotographic marking devices, spectrophotometers may be located downstream of the fusing or drying components of the device, without having to otherwise modify the marking device, or interfere with or interrupt normal marking, or the movement of the marked sheets in the substrate path, and yet provide accurate color measurements of test color patches printed on the moving sheets as they pass the spectrophotometer. Automatic stand-alone color calibration systems enable a complete closed loop color control of a marking device.
Typically, spectrophotometers give color information in terms of measured reflectances or transmittances of light, at different wavelengths of light, from a test surface. A spectrophotometer desirably provides distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels.
Known devices capable of providing distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels include LED based sensors marketed by Color Savvy® or Accuracy Microsensor. LED spectrophotometers, such as those shown in Laser Focus World, June 2003, “Color Sensor Enables Closed Loop Control” by John Wallace and U.S. Pat. Nos. 6,384,918 and 6,633,382, each of which is incorporated herein by reference in its entirety, may be used for color measurement in embedded systems. However, these devices may provide inaccurate spectrophotometric measurements regarding substrates which contain fluorescent whitening agents (FWA).
During the papermaking process, a variety of cleaning and bleaching steps are performed by paper manufacturers on paper pulp in order to increase the whiteness of the paper. Despite cleaning and bleaching, all conventional papers exhibit slightly lower reflectance in the blue region of the spectrum, and, therefore appear to the human eye to be slightly yellow or tan in color. For this reason, most high quality papers intended to be used for color reprographics contain one or more additives generally referred to as fluorescent whitening agents (FWA) or, more generally, “whiteners”. These additives, added early in the papermaking process, absorb light in the ultraviolet (UV) portion of the spectrum (including wavelengths of 330-390 nm) that is reemitted in the visible band, including the blue portion of the spectrum (e.g., at wavelengths of 400-500 nm). This makes the manufactured paper appear whiter, and color images printed thereon appear more saturated and therefore more colorful.
For example, high quality bond papers used in the reprographic industry fluoresce in the blue region when exposed to broadband illumination containing UV. Many conventional LED spectrophotometers contain no UV lamps, and, as a result, the appearance of the bond papers as measured by conventional LED spectrophotometers varies from the appearance of the same bond papers as measured by spectrophotometers with broadband light sources that include ultraviolet.
The Xerox® inline spectrophotometers use an array of 8 light emitting diodes (LEDs) to illuminate test targets printed on paper. LED spectrophotometers have a cost advantage over spectrophotometers using non-LED light sources such as, for example, incandescent lamps and xenon flash lamps, because LEDs are stable, small, low cost, and easily driven, as compared to incandescent lamps and xenon Flash lamps.
Ocean Optics™ provides LED light source spectrophotometers which use Ocean Optics LED light sources, including visible LEDs, or an ultraviolet LED which may be substituted for a visible wavelength LED, and which emits at 380 nanometers; however, Ocean Optics™ discloses using one LED light source in a spectrophotometer light source unit.
The following is an attempt to provide some simplified clarifications relating and distinguishing the respective terms “spectrophotometer,” “colorimeter,” and “densitometer,” as they may be used in the specific context of specification examples of providing components for an on-line color printer color correction system, but not necessarily as claim limitations.
Typical prior spectrophotometers in this context use 16 or 32 channels measuring from approximately 400 nm to 700 nm, to cover the humanly visible color spectra or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. The spectrophotometer desirably provides distinct electrical signals corresponding to the different levels of reflected light from the respective different illumination wavelength ranges or channels.
A “colorimeter” normally has three illumination channels, red, green and blue. That is, generally, a “colorimeter” provides its three (red, green and blue or “RGB”) values as read by a light sensor or detector receiving reflected light from a color test surface sequentially illuminated with red, green and blue illuminators, such as three different color LEDs or one white light lamp with three different color filters. It may thus be considered different from, or a limited special case of, a “spectrophotometer,” in that it provides output color information in the trichromatic quantity known as RGB.
Trichromatic quantities may be used for representing color in three coordinate space through some type of transformation. Other RGB conversions to “device independent color space” (i.e., RGB converted to conventional L*a*b*) typically use a color conversion transformation equation or a “lookup table” system in a known manner.
A “densitometer” typically has only a single channel, and simply measures the amplitude of light reflectivity from the test surface, such as a developed toner test patch on a photoreceptor, at a selected angle over a range of wavelengths, which may be wide or narrow. A single illumination source, such as an IR LED, a visible LED, or an incandescent lamp, may be used. The output of the densitometer detector is programmed to give the optical density of the sample. A densitometer of this type is basically “color blind.” For example, a cyan test patch and magenta test patch could have the same optical densities as seen by the densitometer, but, of course, exhibit different colors.
Thus, a spectrophotometer differs from both a colorimeter and a densitometer.